Monday, July 7, 2008

The polar bear appears to share a common ancestor with the presentday brown bear. It apparently branched off the brown bear lineage during the late Pleistocene. Kurt?en (1964) suggested that ancestors of the modern polar bear were "gigantic." Although still the largest of the extant bears, the polar bear, like many other mammals, has decreased in size since the Pleistocene. Also, significant morphological changes have continued within the last 20,000? to 40,000 years, perhaps through the present (Kurt?en 1964). Stanley (1979) described the many recently derived traits of polar bears as an example of "quantum speciation."

Evidence of polar bear evolution contained in the sparse samples of fossils has been strengthened recently by molecular genetics. Whereas traits of fossil teeth and bones from polar bears clearly indicate their brown bear origins, fossil remains include only a handful of specimens (Kurt?en 1964). Genetic data from extant bears can provide phylogenetic information unavailable in the fossil record. Shields and Kocher (1991) first analyzed mtDNA sequences and showed a close relationship between brown bears and polar bears. Cronin et al. (1991) then discovered that mtDNA of brown bears is paraphyletic with respect to polar bears. That is, the mtDNA of brown bears of the Alexander Archipelago in southeastern Alaska is more closely related to the mtDNA of polar bears than it is to the mtDNA of other brown bears. Cronin et al. (1991) reported that mtDNA sequence divergence between Alexander Archipelago brown bears and polar bears is only about 1%, whereas a divergence of about 2.6% separates polar bears from brown bears occurring elsewhere. Cronin et al. (1991) and Cronin (1993) emphasized that mtDNA sequence divergence trees are not species trees and that mtDNA is not, by itself, a good measure of overall genetic differentiation. Nonetheless, these relationships provide a compelling argument regarding the origin and evolution of polar bears.

Following the discovery of Cronin et al. (1991), others corroborated the finding of paraphyletic mtDNA in brown bears and polar bears. Talbot and Shields (1996a, 1996b) suggested that the Alexander Archipelago brown bears represent descendents of ancestral stock that gave rise to polar bears. This stock may have survived Pleistocene glaciers in an ice-free refugium in southeastern Alaska, isolated from brown bears in other Pleistocene refugia (Heaton et al. 1996). This island-dwelling ancestral stock apparently has remained isolated from the more recent mainland bears by broad ocean passages.

Talbot and Shields (1996b) found mtDNA sequence divergence rates similar to those reported by Cronin et al. (1991), and proposed that ancestors of the Alexander Archipelago brown bears diverged from the other mtDNA lineages of brown bears 550,000? to 700,000 years ago. The mtDNA sequence divergences also suggested that polar bears branched from the Alexander Archepelago ancestral stock of brown bears about 200,000 ?to 250,000 years ago, a date closely corresponding with that suggested in the fossil record (Thenius 1953; Kurt?en 1964). Shields and Kocher (1991) and Cronin et al. (1991) reported that the mtDNA nucleotide sequence divergence between brown and polar bears (grouped together) and black bears was 7?9%. Applying the substitution rate (6%/million years) for mtDNA genes reported by Talbot and Shields (1996a) to the sequence divergence reported by Cronin et al. (1991) suggests that brown bear ancestral stock diverged from that of black bears approximately 1.2 to ?1.5 million years ago. This "molecular clock" estimate may be low. The fossil record suggests black bears diverged from the brown bear lineage 1.5 to ?2.5 million years ago.

Cronin (1993) cautioned that mutation rates vary among genes as well as among taxa, and that conclusions based on "molecular clocks" must be viewed with caution and in the context of other evidence. For example, DNA sequences for two functional nuclear genes, ?-casein and the DQ? gene of the major histocompatability complex, show polyphyletic relationships among the three species of bears (M. Cronin and S. Amstrup, unpublished data). That is, the DNA sequences do not resolve the relationships among the species. These functional genes are presumably under strong selection and do not diverge as rapidly as mtDNA. Nonetheless, the mtDNA analyses indicate that Alexander Archipelogo brown bears derive from more ancient stocks and are more closely related to polar bears than are other members of the brown bear clan. These conclusions also corroborate the recent appearance of the polar bear in the fossil record and the more ancient roots of the black bear (Thenius 1953; Kurt?en 1964). All DNA evidence, regardless of some areas of uncertainty, corroborate conclusions from the fossil record that the polar bear is a recently derived species and is undergoing rapid evolution. The extreme arctic marine environment is undoubtedly exerting strong selection pressures for rapid adaptation.